This invention relates to surface acoustic wave device substrate members, and in particular to the reduction of beam spreading and spurious response in various surface acoustic wave devices.
Surface acoustic wave filters and delay lines can be used to substantial advantage to replace electronic filters and to provide ultra long time delays in ECM and other electronic systems where small size, inexpensive, reliable, reproducible devices are needed. However, diffraction losses (losses due to beam spreading) tend to reduce the efficiency of surface acoustic wave delay lines and filters. The design of such filters becomes difficult when highly apodized transducers are required. Apodized transducers, that is, those having minimum and variable overlap between adjacent interdigital transducer fingers, produce very complex acoustic surface wave diffraction patterns. Design of such devices requires precise knowledge of diffraction losses. Beam spreading, therefor, represents a serious problem with respect to both device design and device efficiency. Commonly used substrate materials employing known surface wave propagation surface cuts often produce surface acoustic wave devices that are subject to excessive diffraction, low coupling coefficient characteristics and high spurious responses. Substantial improvement therefore can be realized in surface acoustic wave devices such as filters, delay lines and the like by improving the surface acoustic wave substrate members. For instance, an ideal surface acoustic wave filter would be approached by using a substrate whose material and propagation surface cut would effect the combination of low diffraction, low spurious response, high coupling, coefficient at high velocity. The present invention is directed toward accomplishing these ends.